1. Field of the Invention
This invention relates to controlling the combining of video signals.
2. Description of the Prior Art
It is often desired to produce a composite picture from two or more pictures represented by respective video signals. For instance, it is often required to inset or key a first picture ("key picture") represented by a first video signal into a second picture ("background picture") represented by a second video signal to form a composite picture. FIG. 1 of the accompanying drawings shows an example of such a composite picture, which comprises a key or foreground picture KP that is keyed or inset into a background picture B. The key picture KP may, again in a manner known per se, be produced by manipulating all or part of digitally stored fields of an input video signal in digital video effects (DVE) equipment. The manipulation may, for example, consist of or include rotation of all or part of a stored picture about one or more of three axes, FIG. 1 showing a case in which a basically rectangular picture having edges E1, E2, E3 and E4 has been manipulated in this way.
The production of a composite picture in this way gives rise to problems along the edges of the key picture. The reason for this resides in the spatial resolution of the pictures. In this regard, both the key picture and the background picture are represented by digital video signals which comprise samples representing respective square picture cells or pixels of the picture. That is, each picture can be considered to comprise an orthogonal array or grid of pixels, each horizontal row thereof being centred on a horizontal scanning line and the horizontal rows being spaced apart by the distance between the scanning lines. (See, in this regard, FIG. 2 of the accompanying drawings, which shows part of a picture divided into pixels P with the scanning lines represented at L.) Thus, the spatial resolution is determined by the pixel size, which is in turn determined by the number of lines per field or frame of the video system employed.
Obviously, in general, when a key picture is mixed into a background picture, the edges of the key picture will not coincide precisely with pixel boundaries. Instead, in general, the edges will intersect pixels. Therefore, when the pictures are keyed together, a decision has to be made on the picture content of each pixel intersected by an edge. Thus, if, for example, the decision is to the effect that each such pixel will comprise either wholly key picture or wholly background picture depending upon whether the majority of that pixel should be occupied by the key picture or background picture, respectively, the result is that the desired boundary between the pictures is in practice provided by a step-wise approximation thereto at pixel resolution. This can be more clearly appreciated by referring further to FIG. 2, in which the desired boundary is shown by a line B1 and the stepwise approximation thereto by a line B2. Thus, the actual edges of the key picture are jagged and there is aliasing between the key and background pictures. The degree of jaggedness becomes particularly noticeable when the edges are close to the horizontal or close to the vertical.
To avoid this type of aliasing, the edges must be defined to greater accuracy and smoothly interpolated. One way of doing this, employed in computer graphics, is to identify, by computation, each pixel crossed by an edge and to compute, for each pixel crossed by an edge, an appropriate value for the intensity of the pixel. It must be appreciated, however, that the edges may intersect with a very large number of pixels whereby the total time taken to perform the necessary computation for all of such pixels may be very long (in some cases in the order of hours), in fact so long that this approach is unfeasible for use in real time processing of video signals.
One approach to solving the above problem that is sufficiently fast for use in real-time video processing is the use of a two-dimensional digital low pass filter. By deriving a value for each pixel of the combined picture by a weighted combination of surrounding pixels, the filter hides the aliasing by, in effect, removing the jaggedness, which represents high frequency picture content. However, using such a filter leads to the disadvantage that the appearance of the edges is "soft", i.e. the edges are not sharply defined. This may be considered subjectively undesirable.
An object of this invention is to enable the combining of video signals in real time in such a manner that the edges of the key picture are processed to avoid (or at least reduce) jaggedness, yet the edge softness encountered by employing filtering is avoided.